This proposal is based on the hypothesis that the increase in adipose tissue mass and the decrease in lean body mass are fundamental phenomena underlying much of the physiological deterioration and the pathological processes that occur with age. It is believed that calorie restriction provides an experimental approach for investigating this hypothesis. It is our assumption that caloric restriction increases longevity and delays pathological processes by preventing physiologic deterioration. If this assumption be correct and if the changes in adipose tissue mass and lean body mass are indeed fundamentally related to mammalian aging, then caloric restriction should modify the way in which lean body mass and adipose tissue mass are affected by aging. Thus, we wish to investigate (1) the effects of a calorically restricted diet on changes in adipose tissue structure and function with age, (2) the effects of calorically restricted diet on lean body mass and on the mass, composition, and physiology of skeletal muscle as a function of age. We also wish to correlate the data obtained on adipose tissue, plasma lipids, muscle and lean body mass with the effects of the calorically restricted diet on longevity. Throughout the course of this research, the rat population will be monitored for the occurrence of disease by means of gross pathologic analysis and by the histopathologic analysis of selected organs. Not only should this research provide fundamental data in regard to the aging process in mammals and point the way for future exploration of the biochemical basis of aging in these animals, but it may also point the way to modifications in nutrition which can be practically brought to bear in developing approaches for increasing the quality of life of the aged.